Why does the glider level off and gain altitude when you pull back on the stick?
A glider has two forms of energy to trade off against each other: Kinetic, and potential. In other words, velocity and altitude.
Stick the nose down, you lose potential energy, and gain kinetic energy.
Pull the nose up, reverse the process.
BTW, by clever seeking of a thermal of rising air, you can steal a bit more potential energy and stay aloft much longer. A powered airplane gets extra energy from its fuel tanks, otherwise it's like a glider.
Bob Hoover used to demonstrate this really well - the tradeoff between potential and kinetic energy in